[ImgBot] Optimize images

*Total -- 609.57kb -> 574.98kb (5.68%)

/docs/_static/images/notebooks_reg-k-means_15_1.png -- 3.10kb -> 2.00kb (35.3%)
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/docs/_static/images/notebooks_skater_12_1.png -- 39.96kb -> 38.37kb (3.97%)
/docs/_static/images/pysal_banner.svg -- 360.54kb -> 360.37kb (0.05%)

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